Electronic – Capacitor Help -> Will stacking of dielectric and metal foil increase capacitance

Sure. In your wimshurst machine example, you're already charging up two capacitors that are built into the machine, they're the jars on the right and left.

If you want to charge up another capacitor, you could connect it between the rod that's going into the jar and the outer bit of metal (though you'd probably need a capacitor for each jar), or charge it by putting it between the balls. However, you need to make sure the capacitor is rated to the voltage you're charging it to (on the order of 10kV), which is not common. Since you're making it yourself, you will need to use a thickish dielectric and be prepared for it to arc over. You also need to be prepared for it to take a long time to charge. According to wikipedia, a leyden jar has about 1nf of capacitance, so if you add say a 10nF capacitor to each jar, the sparks will occur about 10x less often.

MLCCs tend to have a capacitance drop in high DC voltages, which means lower currents will flow!

I do not think that that is an effect that you would want to rely on for your design. This effect is an unwanted effect of the capacitor, it might not behave as expected/specified. Also the effect will probably not "do" enough (be strong enough) to get the effect that you want / are hoping for.

I would definitely go for those "safety caps". They're designed short the RF noise between AC mains and a "safe to touch" chassis. You need the Y-type as that should fail as an open. That would increase RF emissions but not make the chassis carry 500 V AC.

The X-type fail short which then shorts the 500 VAC to the chassis: not what you want!

The 250 VAC rating could indeed be a problem, it is unclear from the datasheet how these capacitors would respond to a constant 500 VAC.

If you cannot find 500 VAC rated Y-type safety capacitors then you could consider using two capacitors in series. I would then also add a high value resistor (like 1 Mohm or more) in parallel with each capacitor to make sure the voltage is divided equally, like so:

Realize that you need resistors which can handle 250 V AC, the larger SMD types should suffice, for smaller types consider using two (or more) resistors in series to divide the voltage.